The present invention relates to a fluid flow rate detecting technique, and particularly to a thermal flowmeter having a fluid discriminate function.
Various types have been known for a flowmeter [flow rate sensor] (or current meter [flow velocity sensor]) for measuring or detecting the flow rate (or flow velocity) of various kinds of fluid, particularly liquid. Of these types of flowmeters, a so-called thermal type (particularly, indirectly heated type) flowmeter has been used because the price thereof is lower.
One of indirectly heated type flowmeters is designed and used so that a sensor chip comprising a thin-film heating element and a thin-film temperature sensing element which are laminated on a substrate through an insulating layer by using the thin film technique is disposed so as to enable the heat transfer between the sensor chip and fluid flowing in a pipe. The electrical characteristic of the temperature sensing element, for example, the value of the electrical resistance is varied by supplying current to the heating element to heat the temperature sensing element. The variation of the electrical resistance value (based on increase of the temperature of the temperature sensing element) is varied in accordance with the flow rate (flow velocity) of the fluid flowing in the pipe. This is because a part of the heating value of the heating element is transferred into the fluid, the heating value thus diffused into the fluid is varied in accordance with the flow rate (flow velocity), and the heating value supplied to the temperature sensing element is varied in accordance with the variation of the heating value diffused into the fluid, so that the electrical resistance value of the temperature sensing element is varied. The variation of the electrical resistance value of the temperature sensing element is also varied in accordance with the temperature of the fluid. Therefore, a temperature sensing element for temperature compensation is installed in an electrical circuit for measuring or detecting the variation of the electrical resistance value of the temperature sensing element to reduce the variation of the flow rate measurement value due to the temperature of the fluid as much as possible.
With respect to the indirectly heated type flowmeter using the thin-film element as described above, JP(A)-11-118566 discloses an example of the indirectly heated type flowmeter. The flowmeter disclosed in this publication uses an electrical circuit (detection circuit) containing a bridge circuit to achieve the electrical output corresponding to a flow rate of the fluid.
In such a flowmeter, a value of the output of the detection circuit varies in accordance with thermal property of the fluid, even if actual flow rate value is equal. Therefore, in general, assuming that the fluid to be subjected to the flow rate detection is a required one, a calibration curve for the required fluid is used to convert the output value of the detection circuit to the flow rate value.
Recently, using an itemized fluid source obtained by dividing the same kind of fluid to be subjected to the flow rate detection, the flow rate is detected one by one for a plurality of itemized fluid sources.
For example, in synthesis of reagent of high degree of purity, synthesis of medicine, chemical analysis or the like, a small portable vessel containing a raw material fluid or a reagent fluid is connected to a reactor or an analyzer through a fluid flow passage, and the raw material fluid or the reagent fluid is fed to the reactor or the analyzer, while the flow rate of the fluid is detected at the fluid flow passage. When supplementing the raw material fluid or the reagent fluid, a vacant small portable vessel is replaced with a fresh small portable vessel filled with the raw material fluid or the reagent fluid to be connected to the reactor or the analyzer.
Furthermore, when a medical fluid is injected into a living body, the medical fluid is itemized so as to be filled up in portable medical fluid packs. A portable medical fluid pack is connected to the living body, for example a blood vessel thereof, through a fluid flow passage, and the medical fluid is injected into the living body, while the flow rate of the medical fluid is detected at the fluid flow passage. When supplementing the medical fluid, a vacant portable medical fluid pack is replaced with a fresh portable medical fluid pack filled with the medical fluid to be connected to the living body.
Utilization of the above itemized fluid source practically provides great advantage, however, there is a possibility of causing a mistake that a vacant itemized fluid source is replaced with a fresh itemized fluid source filled with non-required fluid when exchanging the itemized fluid source. In such a case, when the fluid is fed through the fluid flow passage, accurate flow rate detection cannot be conducted due to the difference in thermal property between the required fluid and the actually fed non-required fluid, and furthermore the incorrect fluid feeding causes an inferiority or accident in manufacturing or analysis or a medical accident.
Therefore, the present invention has an object to provide a thermal flowmeter having a function of discriminating whether the fluid to be subjected to the flow rate detection is the required one or not with simple constitution, in order to avoid the above-mentioned incorrect fluid flowing.
In order to attain the above objects, according to the present invention, there is provided a flowmeter, comprising:
a flow rate detecting unit containing a heating element, a flow rate detecting temperature sensing element and a flow rate detecting electroconductive heat transfer member extending into a fluid flow passage, which are disposed so as to enable heat transfer therebetween, the flow rate detecting temperature sensing element varying in electrical characteristic value in accordance with flow of a fluid in the fluid flow passage through heat exchange with the fluid in the fluid flow passage which is carried out through the flow rate detecting electroconductive heat transfer member; and
a fluid temperature detecting unit containing a fluid temperature detecting temperature sensing element and a fluid temperature detecting electroconductive heat transfer member extending into the fluid flow passage, which are disposed so as to enable heat transfer therebetween, the fluid temperature detecting temperature sensing element varying in electrical characteristic value in accordance with the temperature of the fluid through heat exchange with the fluid in the fluid flow passage,
wherein a flow rate of the fluid is detected on the basis of the electrical characteristic value of the flow rate detecting temperature sensing element and the electrical characteristic value of the fluid temperature detecting temperature sensing element, and fluid discrimination is effected by determining a conductivity between the flow rate detecting electroconductive heat transfer member and the fluid temperature detecting electroconductive heat transfer member.
In an aspect of the present invention, the fluid discrimination is effected by judging the fluid as a required one in case that a detected value of current flowing between the flow rate detecting electroconductive heat transfer member and the fluid temperature detecting electroconductive heat transfer member therethrough is a value within a predetermined range for the required fluid, while judging the fluid as not the required one in case that the detected value of current is a value without the predetermined range for the required fluid.
In an aspect of the present invention, the flow rate of the fluid is detected on the basis of an output of a detection circuit containing the flow rate detecting temperature sensing element and the fluid temperature detecting temperature sensing element and a calibration curve.
In an aspect of the present invention, a portion of the flow rate detecting electroconductive heat transfer member constitutes an electrode terminal of the flow rate detecting unit, and a portion of the fluid temperature detecting electroconductive heat transfer member constitutes an electrode terminal of the fluid temperature detecting unit.